1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device, with a gap supporting part that prevents defects caused by gravity.
2. Discussion of the Related Art
With the development of an information oriented society, the demands on display devices have increased. To meet the demands, different flat display devices have recently been developed for use in various devices, such as the Liquid Crystal Display Device (LCD), the Plasma Display Panel (PDP), the Electro Luminescent Display (ELD), and the Vacuum Fluorescent Display (VFD).
Among the flat display devices, LCD devices have been used the most widely as portable display devices replacing the Cathode Ray Tube (CRT) because LCD devices have excellent picture quality, are light weight, are thin, and have low power consumption. In addition to portable LCD devices, LCD devices are under development for televisions and computer monitors.
The LCD device has a liquid crystal panel for displaying a picture and a driver that provides a drive signal for the liquid crystal panel. The liquid crystal panel has a lower substrate and an upper substrate opposite to each other with a gap between the substrates and a liquid crystal layer between the two substrates. The liquid crystal panel displays an image by controlling the light transmittivity of the liquid crystal with an electric field formed between the two substrates.
The structure and operation of a related art liquid crystal panel in an LCD device will be described briefly with reference to FIG. 1. In FIG. 1, the liquid crystal panel has a lower substrate 22 bonded to an upper substrate 5 with a gap between the substrates and a liquid crystal layer 14 injected between the lower substrate 22 and the upper substrate 5.
The lower substrate 22 has a plurality of gate lines 13 arranged in one direction at fixed intervals and a plurality of data lines 15 arranged in a direction substantially perpendicular to the gate lines at fixed intervals that define a pixel regions P. The lower substrate 22 has a plurality of pixel electrodes 17 in pixel regions P defined by the gate lines and the data lines, and a plurality of thin film transistors T are formed at the cross parts of the gate lines 13 and the data lines 15.
The upper substrate 5 has a black matrix layer 6 for shielding the thin film transistors T, gate lines 13, and data lines 15 from light. The upper substrate also has a R, G, B color filter layer 8 for displaying colors and a common electrode 18 for implementing a picture.
The thin film transistor T has a gate electrode projecting from the gate line 13, a gate insulating film (not shown) formed on the entire surface, an active layer on the gate insulating film over the gate electrode, a source electrode projecting from the data line 15, and a drain electrode arranged opposite to the source electrode. The pixel electrode 17 is formed of a transparent conductive metal such as indium-tin-oxide (ITO).
The LCD device can display a picture by controlling the amount of light passing through the liquid crystal layer 14 by changing the orientation of the liquid crystal layer 14. The orientation of the liquid crystal layer at the pixel electrode 17 is set according to a signal from the thin film transistor T.
A related art method for fabricating the liquid crystal panel will be described, briefly. FIG. 2 illustrates a flow chart of the steps for a related art method of fabricating a liquid crystal panel.
A plurality of gate lines with gate electrodes are formed in one direction on a panel region of a large sized glass substrate, which is used for forming a plurality of panels. Next, a gate insulating film is deposited on an entire surface, an active layer is formed on the gate insulating film over the gate electrode, and a plurality of data lines are formed in a direction substantially perpendicular to the gate lines such that source and drain electrodes are disposed on opposite ends of the active layer. Then, a protection film is formed on the entire surface, a contact hole to the drain electrode is formed, and a pixel electrode is formed in the pixel region, to form a thin film transistor array (st1).
A polymer thin film is deposited on the substrate with the thin film transistor array and aligned by rubbing or optical alignment (st2). The rubbing or optical alignment process fixes the initial orientation of the liquid crystal, enables regular driving of the liquid crystal, and provides uniform display performance. In general, the alignment film is typically formed of an organic polymide group.
A seal pattern for bonding the substrates is printed on the periphery of each panel region of the glass substrate having the alignment film formed thereon (st3). The seal pattern serves to form the gap required for injection of the liquid crystal and prevents leakage of the liquid crystal injected therein. The printing of the seal pattern is a process of screen printing a desired fixed pattern of a thermo-setting resin.
Then, spacers are spread uniformly on the substrate to maintain a fixed cell gap (st4). Fixed size spacers are used to maintain an accurate and consistent gap between the substrates. The spaces should be uniformly distributed on the lower substrate. The spacers may be spread by a wet spreading method in which the spacers are spread mixed with alcohol or by a dry spreading method in which only the spacers are spread. Two examples of dry spreading are an electrostatic method in which static electricity is used and an antistatic method in which a gas pressure is used. The antistatic method is mostly used for liquid crystal panels having a structure vulnerable to static electricity.
A black matrix layer is formed on another glass substrate, which is to be bonded with a substrate having the thin film transistors formed thereon. The black matrix shields portions of the substrate outside of the pixel region. Next, the R, G, B color filter layer is formed on each pixel region, and a common electrode is formed on the entire surface. Then, an alignment film is formed over the common electrode.
When the spacer spreading is finished, the substrate having the thin film transistor array formed thereon and the substrate having the color filter array formed thereon are bonded (st5). The method of bonding the thin film transistor array substrate and the color filter array substrate depends on the gap tolerance which typically is a few micrometers. A bonding error may allow light leakage which will degrade the picture quality of the liquid crystal cell.
Because the bonded substrates have a plurality of panels, the bonded substrates are cut into unit panels (st6). The cell is cut by scribing a cutting line on a substrate surface with a pen of diamond having hardness greater than the glass substrate and breaking the bonded substrates along the cutting line by applying a force.
Next, liquid crystal is injected into the space between the bonded substrates, and the liquid crystal injection hole is sealed (st7). A liquid crystal panel has a few hundred square centimeters of area and a gap of a few micrometers. Therefore, to effectively inject the liquid crystal into such a panel, a vacuum injection method is used in which a pressure difference between inside/outside of the panel forces liquid crystal into the gap.
The methods for spreading spacers described above have limitations. Consequently, there has been much research on methods of forming spacer patterns during the fabrication of the substrate. One method suggests creating a pattern of spacers made of an organic material on an upper substrate.
FIGS. 3A to 3F illustrate cross-sections of a panel showing the steps of a related art method of forming a spacer pattern on a color filter array substrate.
Referring to FIG. 3A, a light shielding metal is deposited on a transparent insulating substrate 5 and portions are removed to form a black matrix layer 6 to shield light from portions of the liquid crystal panel outside the pixel regions. The black matrix layer 6 may be a thin metal film of chrome Cr having an optical density greater than 3.5, a film of organic material or a bi-layered black matrix of chrome Cr/chrome oxide CrOx. Accordingly, the black matrix is formed of one of above materials depending on the application.
Referring to FIG. 3B, the color filter layer 8a, 8b, 8c is formed on respective pixel regions, of R, G, B color resins. The main ingredients of the color resin are a photopolymerization type photosensitive composition of a photopolymerization initiator, a monomer, a binder, and organic pigments having colors of red/green/blue. A coat of the red color resin is formed on the entire surface of the substrate 5 having the black matrix formed thereon and removed selectively to form a red color filter layer 8a in the desired region. Next, a coat of a green color resin is formed on the entire surface of the substrate 5 having the red color filter layer 8a formed thereon and removed selectively to form a green color filter layer 8b in the desired region. Finally, a coat of blue color resin is formed on the entire surface of the substrate 5 having the red and green color filter layers 8a and 8b formed thereon and removed selectively to form a blue color filter layer 8c in the desired region.
Referring to FIG. 3C, a coat of transparent resin with an insulating property is formed on the substrate 5 to form an overcoat layer 26. The coat of transparent resin forms a flat surface over the color filter layers 8a, 8b and 8c. 
In FIG. 3D, a common electrode 18 of a transparent metal, such as indium-tin-oxide (ITO) and indium-zinc-oxide (IZO), is formed on the overcoat layer. A common voltage will flow through the common electrode 18 to drive the liquid crystal 14 together with a pixel voltage that will flow through the pixel electrode 17 on the thin film transistor array substrate 22.
Next, in FIG. 3E, a transparent organic film is formed on the entire surface of the substrate 5 having the common electrode 18 formed thereon and subjected to photolithography and etching to form spacers 20 of required heights.
Referring to FIG. 3F, after the spacers 20 are formed, the entire surface of the substrate 5 is coated with a transparent organic insulating material such as polyimide to form an alignment film 22. Then, the surface of the alignment film 22 is rubbed in a predetermined direction. Thus, a related art color filter array substrate can be fabricated.
Alternatively the spacers 20 may be are patterned after the alignment film 22 is formed, but the alignment film 22 under the spacers 20 may be damaged by chemical used for patterning the spacers 20. Therefore, the alignment film 22 typically is formed after the spacers 20 are patterned.
FIG. 4 schematically illustrates the cross-section of a bonded thin film transistor array substrate and color filter array substrate with spacers formed thereon in a vertical position according to the related art. LCD devices are mostly employed as monitors, which are used in a vertical position when the LCD devices are in notebook computers or general LCD monitors. Also, during testing the LCD panel is placed in a vertical position. When the LCD panel is vertical the liquid crystal concentrates in the lower end due to gravity resulting in a difference in the liquid crystal concentration in the upper end and the lower end of the LCD panel. The higher liquid crystal concentration at the lower end causes the cell gap between the upper substrate 5 and the lower substrate 22 to vary from top to bottom which results in the cell gap between the substrates to become greater than the height of the column spacers 20 at the lower end of the LCD panel.
The uneven cell gap between the substrates and the separation of the column spacers 20 from the substrate in the lower side causes the picture quality to differ between the upper end and the lower end. This problem increases as the size of the LCD panel increases.